Coaxial cable networks are a common medium that is used to distribute video and audio programming to consumers with a higher quality than is typically achieved using conventional antennas connected to a television at each user's location. A cable network typically includes a head end that receives inputs or programming from a number of content providers, e.g., ABC, NBC, CBS, Fox, CNN, ESPN, etc. The head end is typically connected to a distribution network that distributes the programming from the head end to, for example, the television sets of a number of end users. The distribution network can include coaxial cable alone or in combination with optical fiber, or other distribution medium.
Radio frequency (RF) signals transmitted over a coaxial cable portion of the distribution network tend to attenuate as a function of distance from the head end. This means that the RF signals decrease in quality as they get further away from the head end. To compensate for this attenuation, cable networks typically include network amplifiers that are selectively distributed throughout the network. These amplifiers receive and amplify the RF signals at these selected points in the network such that the signals provided to each end user provide an acceptable level of quality.
Network amplifiers typically include two or more "monitoring circuits" that allow a network operator to monitor RF signals transmitted over the network and within the amplifier. Conventionally, a monitoring circuit uses a directional coupler that is placed within the RF signal path of the amplifier. The directional coupler includes a transformer, such as a double aperture balun device. The directional coupler couples monitored RF signals from the amplifier's RF signal path to, e.g., a test point connector, or an input of another circuit. The monitored RF signals are typically scaled down by a selected factor at the test point connector, e.g., 20 decibel (dB).
Network amplifiers conventionally include a monitoring circuit that is associated with the input port and a monitoring circuit that is associated with the output port of the amplifier. Further, the amplifier may also include other monitoring circuits that are used to provide internal feedback signals to circuits of the network amplifier such as an automatic gain control (AGC) circuit. The AGC circuit adjusts the level of the gain of the network amplifier so that the output is within an acceptable range.
One drawback with conventional monitoring circuits is a 1 dB maximum insertion loss created in the RF signal path of the amplifier by the monitoring circuit. This means that the monitoring circuit reduces or attenuates the RF signal output by the amplifier. This loss in signal strength reduces the effectiveness of the amplifier. This insertion loss is additive and applies to each monitoring circuit. Thus, a network amplifier with input, output and AGC monitoring circuits produce 3 dB reduction in signal strength. Additional monitoring circuits further increase the RF signal reduction.
Conventional attempts to overcome the RF signal reduction can add to or complicate other problems with the amplifier. For example, simply increasing the gain of the amplifier can be used to compensate for the 1 dB reduction caused by a monitoring circuit. However, the intermodulation distortion of the amplifier increases by 2 to 3 dB for every 1 dB in increase of the output level of the network amplifier. The intermodulation distortion is important to the perceived video fidelity of the composite signal in the cable network.
Use of a conventional monitoring circuit at the input of a network amplifier can add up to 1 dB of noise figure to the preamplifier noise figure for the network amplifier. Operating noise figure is an important factor is determining where to place amplifiers in a given network.
One additional problem with conventional monitoring circuits is that the directional couplers used have a limited bandwidth. That is, the directional coupler can only handle signals over a limited frequency range; typically, 5 to 860 MHZ with .+-.0.25 dB flatness. Currently, cable networks plan to expand the frequency range of the channels offered to go as high as 1000 MHZ. This will make the directional coupler a limiting factor in monitoring circuits.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a circuit for monitoring RF signals in a network amplifier with reduced insertion loss and higher bandwidth capability.